The permeability to air is of major importance for the functioning of many technical fabrics for parachutes or sailcloth. It is particularly relevant to air bags. Such technical fabrics are known to skilled artisans. The low permeability to air required of these fabrics has hitherto been obtained by the application of a coating.
A significant disadvantage to the use of coated fabrics, which applies particularly for the production of air bags, is that a coated fabric has a volume when folded at least 10% higher than that of an uncoated fabric. The space required for accommodating the air bag, for example in a steering wheel, is thus greater for a coated fabric than for an uncoated fabric. One particular disadvantage arises from the necessity to cover the coating with talc to prevent the coatings which are in contact with one another in the folded air bag from sticking together. The talc escaping from the air bag when the air bag function is released then causes discomfort to the passengers. Moreover, air bags of uncoated fabrics have a much lower mass than those of coated fabrics so that the steering wheel is easier to handle.
It has therefore been attempted to avoid the necessity of applying a coating. One possible solution for air bag fabrics is proposed in EP-A 314,867, in which the air permeability is adjusted to the required low value by successive shrinkage, thermofixing and calendering. These treatment stages constitute a finishing process which is not only very inconvenient but also costly. Moreover, the fabric described in EP-A 314,867 is hardly suitable for the production of air bags since the asymmetric fabric structure provided (different thread counts in warp and weft) cannot meet the demand of motor car manufacturers for equal strength in both thread directions. This is a necessary condition since the radially symmetrical air bag structure has no preferential direction.
Another solution is proposed in Canadian Patent No. 974,745. In this case, asymmetric fabrics with a very dense weave produced from heat shrinkable synthetic fiber yarns are subjected to a dry heat treatment, preferably in a tentering frame. Apart from the high energy costs, the fabrics described here also fail to meet the demand of motor car manufacturers for equal strength in warp and weft in air bags on account of the chosen asymmetric fabric structure.
A process for the production of fabrics using extremely high shrinking polyester fibers is described in EP-A 336,507. The shrinkage is permitted and the fabric structure made dense for adjustment to a low permeability to air and water by means of a wet treatment followed by a thermofixing treatment. This process may also be used for technical fabrics, but it is unsuitable for most articles in this field, especially for air bag fabrics, as the high shrinkage polyester yarns used are not sufficiently strong for technical fabrics. Moreover, the thermofixing process increases the production costs and even has an adverse effect on the air permeability.
The problem therefore arose of developing an inexpensive process for the production of technical fabrics which would not only provide the required air permeability but also fulfill the requirement for strength in these fabrics. Such fabrics are of particular interest for air bags.
The term "air bag fabrics" is used hereinafter to denote in particular the fabrics for the contact part of the two-part air bag, which are required to have a particularly low air permeability of &lt;10 l/dm.sup.2.min at 500 Pa test difference pressure. The two-part air bag has a filter part of higher air permeability to enable the air flowing into the air bag on release of the air bag function to escape. In a one-part air bag, openings are punched into the air bag to enable the air to escape. The required low air permeability in this case applies to the entire fabric used for the air bag.